This invention relates to thermosensitive recording materials which have good thermal stabilities and are suitable for cards to be used for a relatively long period of time such as season tickets or passes and credit cards.
In recent years, thermosensitive recording systems have been widely employed as output recording systems such as those for transmission-information recording (e.g., facsimile), output recording in electronic computers, and output recording in various testers, as well as printer systems in automatic ticket machines. Moreover, the utilization of such systems for issuing season tickets, credit cards, various commutation tickets or passes and the like to be used for relatively long periods (hereinafter generally referred to as "long-term cards" is also coming under study. This is because thermosensitive recording systems have such features as their affording recordings in which the information contents can be directly viewed by eye and simpler maintenance, and more compact and less expensive apparatus in comparison with other printer systems such as ribbon printers.
As recording materials to be employed in such thermosensitive recording systems, recording materials in each of which a thermosensitive layer comprising a color former such as leuco-dyes and a developer such as phenolic compounds dispersed in a solid vehicle is formed on a substrate have been generally used. When the thermosensitive layer is heat-printed by a thermal head or a heating pen, the color former and/or the developer contained in the binder (solid vehicle) is melted or sublimated in the heat-printed portions, and both the color former and developer are closely contacted and reacted to produce color.
There are some problems, however, in the use of such a recording material for long-term cards as mentioned above because of inadequate thermal stabilities principally in the course of storage after or before issuing of the cards. More specifically, such long-term cards must maintain clear recordings in their thermosensitive layers during their period of use after issuance, and therefore they are required to have no fogging in the thermosensitive layer and no fading of printed recording. Because the long-term cards are expected to be exposed to a temperature as high as 80.degree. C. especially in summer (for example, when they are left in automobiles), the above mentioned requirement must be satisfied also under such a high temperature condition.
Furthermore, a thermosensitive recording apparatus is relatively inexpensive and compact, and consequently may be installed at a place where temperature conditions are relatively unfavorable, for example, in front of shops or stations. Thus, unprinted recording materials corresponding to before-issue long-term cards are also desired to neither produce color at a high temperature of about 80.degree. C. nor decrease in color-producing performance at the time of recording.
As described above, the thermosensitive recording materials to be used for long-term cards are required to have particularly excellent thermal resistance, but the conventional thermosensitive recording materials have not always satisfied this requirement.
To solve this problem, there have been proposed a method for decreasing the color-producing sensitivity and raising the color-producing temperature by lowering the concentration of a color former or a developer contained in the thermosensitive layer and a method of adding to a thermosensitive layer a water-insoluble alkaline inorganic pigment such as calcium carbonate or magnesium carbonate (Japanese Laid-open Patent Publication No. 6077/1980).
Such methods, however, are not effective in preventing fading of printed recordings or decrease in color-producing performance when unprinted recording materials are stored.